CS-symmetric triarylborane substituted bisthiazole for selective detection of F?and CN?ions

Article abstract of DOI:10.1016/j.tetlet.2016.07.051

Triarylborane substituted bisthiazole 1 was designed and synthesized by the Sonogashira cross-coupling reaction for selective detection of F^?and CN^?ions in the presence of Cl^?, Br^?, I^?and NO₂^?anions. The detection limits for F^?and CN^?were found to be 5.7?×?10^?6?M and 2.1?×?10^?6?M, respectively. The time dependent density functional theory (TD-DFT) calculation at B3LYP on the bisthiazole triarylborane 1 was studied for F^?and CN^?ions. In the case of 1 Boron is electron deficient, the addition of F^?ion should increase the electron density on Boron which was confirmed by electrostatic potential surfaces of bisthiazole triarylborane dyad 1, 1-F.

Full text of DOI:10.1016/j.tetlet.2016.07.051

Accepted Manuscript
C -Symmetric Triarylborane Substituted Bisthiazole for Selective Detection of
S
−
−
and CN ions
F
Ramesh Maragani, T. Sheshashena Reddy, Rajneesh Misra
PII:
S0040-4039(16)30896-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.07.051
TETL 47913
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
9 May 2016
9 July 2016
14 July 2016
Please cite this article as: Maragani, R., Sheshashena Reddy, T., Misra, R., C -Symmetric Triarylborane Substituted
S
−
Bisthiazole for Selective Detection of F and CN ions, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/
−
j.tetlet.2016.07.051
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−
−
C
S
-Symmetric Triarylborane Substituted Bisthiazole for Selective Detection of F and CN ions
Ramesh Maragani, T. Sheshashena Reddy, Rajneesh Misra*

1
Tetrahedron Letters
jour nal homepage: w ww.elsevier. com
−
-Symmetric Triarylborane Substituted Bisthiazole for Selective Detection of F and CN ions
−
C
S
Ramesh Maragani, T. Sheshashena Reddy, Rajneesh Misra*
1
Department of Chemistry, Indian Institute of Technology Indore, MP 452017, India.
ARTICLE INFO
ABSTRACT
Triarylborane substituted bisthiazole 1 was designed and synthesized by the Sonogashira
Article history:
Received
−
−
−
cross–coupling reaction for selective detection of F and CN ions in the presence of Cl ,
Received in revised form
Accepted
−
Br , I and NO
−
−
−
anions. The detection limits for F and CN was found to be 5.7 ×10
−
−6
2
M
−6
and 2.1×10 M respectively. The time dependent density functional theory (TD-DFT)
Available online
−
−
calculation at B3LYP on the bisthiazole triarylborane 1 was studied for F and CN ions. In
−
the case of 1 Boron is electron deficient, the addition of F ion should increase the electron
density on Boron which was confirmed by electrostatic potential surfaces of bisthiazole
Keywords:
triarylborane dyad 1, 1-F.
Anion sensing, Photophysical, Computational, ESP
properties
In recent years considerable effort has been made in the design and
synthesis of anion probes due to their application in wide range of
1
chemical, environmental, industrial and biological processes. The
The bisthiazole is a strong acceptor and bisthiazole systems are
studied in Organic photovoltaics (OPVs), dye sensitized solar cells
24
(DSSC). In triarylborane substituted bisthiazole the bisthiazole
fluoride ion is primarily used in dental care and the treatment of
,3
drags electron density from triarylborane unit and increases its Lewis
acidity. In this contribution, we report triarylborane functionalized
2
osteoporosis. However, excessive amounts of fluoride ions in
human body result in dental and skeletal injuries, bone diseases,
−
bisthiazole for the selective detection of F and CN anions.
−
4
lesions of the thyroid, liver and other organs. On the other hand
The triarylborane substituted bisthiazole 1 was synthesized by the
Pd–catalysed Sonogashira cross–coupling reaction of bisthiazole as
shown in Scheme 1. The starting material 5, 5’-dibromo-4,4'-
dihexyl-[2,2']bithiazolyl (d) and (4–ethynylphenyl)–dimesitylborane
cyanide ion is widely used in synthetic fertilizers, dyes and textiles,
petrochemical elution, pesticides, automobiles, pharmaceuticals and
5
electroplating. Therefore there is a need of sensor which can detect
25, 26
these anions. The literature reveals that a variety of triarylborane
6
based molecular systems has been explored in organic electronics.
(e) was synthesized by the reported procedures.
The triarylborane substituted bisthiazole 1 was synthesized by the
Sonogashira cross–coupling reaction of bisthiazole d with (4–
π
The triarylborane containing molecular systems have empty p –
orbital on the boron centre which can effectively communicate with
7
3 4
ethynylphenyl)dimesitylborane e using the Pd(PPh ) as a catalyst in
the neighboring π-electron system. This makes triarylborane
derivatives a potential candidate for optoelectronics applications.
tetrahydrofuran (THF) solvent, triethylamine as a base at 100 °C for
24h, which resulted the triarylborane substituted bisthiazole 1 in 74%
yield (Scheme 1). The triarylborane substituted bisthiazole 1 was
8
-21
The triarylborane molecular systems exhibit high Lewis acidity and
steric hindrance due to the bulky mesityl groups. The acidity of the
triarylborane moiety can be tuned by substituting the electron
1
13
characterised by H NMR, C NMR and HRMS techniques. In the
1
H NMR spectra of triarylborane substituted bisthiazole 1, the
22
donating and electron withdrawing groups.
The electron-
aromatic protons of triarylborane exhibit multiplet between 7.49 –
7.54 ppm and one singlet at 6.84 ppm.
withdrawing group conjugated with the triarylborane increases the
Lewis acidity of the triarylborane. Our research group is involved in
the design and synthesis of anion sensors based on triarylborane
B
−
−
containing molecular systems. We have reported the F and CN ion
sensing behavior of triarylborane functionalized naphthalimide,
2
porphyrin, and pyrazabole (Chart 1).
Br
N
e
B
N
S
S
S
S
3
Br
THF/Et3N
Pd(PPh3)4
00 oC, 24 h
B
N
N
d
1
1
Bun
N
nBu
B
B
B
O
N
O
N
N
B
B
N
Scheme 1. Synthesis of triarylborane substituted bisthiazole dyad 1.
Bun nBu
N
N
N
N
Zn
The electronic absorption and emission spectra of 1 was recorded in
THF. The triarylborane substituted bisthiazole 1 show red shifted
absorption band at 416 nm compared to the bisthiazole intermediate
d (353 nm) which can be attributed to the π–π* electronic transition.
B
—Cha—rt—1. Structures of previously reported triarylborane systems.

2
The triarylborane substituted bisthiazole 1 show emission maximum
Fig 2. (a) Absorption and (b) fluorescence titration spectra of triarylborane
−5
substituted bisthiazole 1 (2.1×10 M) with TBAF in THF solvent.
at 474 nm and 503 nm and the quantum yield of 0.26 (Fig 1).
After addition of cyanide ion, there was a gradual decrease in the
intensity of the absorption band at 416 nm and a new band at 510 nm
was developed (Fig 3). One isosbestic point was observed at 453 nm.
Job’s plot analysis at 416 nm revealed a 1:2 stoichiometry between
triarylborane substituted bisthiazole 1 and cyanide ions (Fig S9).
Upon addition of cyanide ion to 1, its emission intensities at 474 nm
and 503 nm gradually decreased and become a broad emission peak
centering at 506 nm. The fluorescence was quenched upto 2.9 folds
(
Fig 3) and fluorescence quantum yield of formed complex is 0.08.
To investigate the selectivity of 1 towards cyanide, the changes in
the absorbance and emission spectra of 1 was monitored in the
−
−
−
−
presence of other competing anions such as Cl , Br , I , NO
−
2
and F
(
Fig S10). Expect for
Fig 1. Normalized electronic absorption and emission spectra (λex 416 nm) of
triarylborane substituted bisthiazole 1 (2.1×10 M) in THF solvent.
−5
The anion sensing ability of the triarylborane substituted bisthiazole
1
was investigated by the absorption and emission studies, using
−
−
−
−
−
various anions (F , Cl , Br , I , NO
−
2
and CN ). The anions were
used in their tetrabutylammonium salts in THF solvent. The
absorption and emission studies show no significant change in the
−
presence of Cl , Br , I and NO
−
−
−
2
anions (Fig S5 and Fig S6), while
cyanide, no significant changes could be observed upon addition of
−
−
−
−
other anions (5 equiv.).The detection limit of 1 for F and CN was
F and CN exhibit profound effect. Upon addition of fluoride ion the
intensity of absorption band at 416 nm gradually decreased and
shifted to 429 nm (Fig 2). One isobestic point was observed at 446
nm. The bisthiazole triarylborane dyad 1 exhibits fluorescence at 474
nm and 503 nm upon excitation at 410 nm in THF solvent. Job’s plot
calculated based on the fluorescence titration data and found to be
−
5.7 ×10 M and 2.1×10 M respectively.
6
−6
(a)
(b)
Fig 3. (a) Absorption and (b) fluorescence titration spectra of bisthiazole
triarylborane 1 (2.1×10 M) with TBACN in THF solvent.
analysis at 416 nm revealed
a 1:2 stiochiometry between
−5
triarylborane substituted bisthiazole 1 and fluoride ions (Fig S7).The
−
emission studies show no significant change in the presence of Cl ,
−
Br , I and NO
−
−
2
−
anions (Fig S5), whereas F and CN show
−
The binding constant of the complex formed in solution was
estimated by using the standard Benesi−Hildebrand equation 1.
significant effect. Upon addition of fluoride ion to 1, its emission
intensities at 474 nm and 503 nm gradually decreased and become a
−
broad emission centred at 511 nm. The addition of F ion results in
Where I
0
is the intensity before addition of anion, I is the intensity in
−
the presence of F / CN , I
−
1
is intensity upon saturation with anion,
red shifted in emission at 511 nm and fluorescence was quenched by
2
.3 folds (Fig 2) and fluorescence quantum yield of formed complex
is 0.12. To investigate the selectivity of 1 towards fluoride, the
changes in the absorbance and emission spectra of 1 was monitored
and K
a
is the binding constant of the complex formed. The binding
−
in the presence of other competing anions such as Cl , Br , I , NO
−
−
−
2
−
constant towards the F and CN ion were found 11,100 M and
−
–1
−
and CN (Fig S8). Except for fluoride, no significant changes could
be observed upon addition of other anions (5 equiv.).
–1
2,464 M using the above equation (Fig S10 and Fig S11).
1
−
In order to gain further insight into the F and CN binding, H-NMR
−
1
1
titration studies were performed on 1. The comparison of H–NMR
spectra of 1 before and after addition of fluoride anion is shown in
Fig 4. The aryl protons of triarylborane show multiplet between 7.49
–
7.54 ppm and one singlet 6.84 ppm (Chart S1). From the chart S1
and Fig 4 it can be observed the phenyl protons (H-1) singlet signal
at 6.84 ppm are shielded upon addition of fluoride anion and the new
signal was observed at 6.57 ppm. In the case of cyanide sensing aryl
protons of triarylborane singlet signal at 6.84 ppm was shielded upon
addition of cyanide anion and it was observed at 6.59 ppm (Fig S13).
(a)
(b)

3
1
Fig 5. The electrostatic potential surfaces of bisthiazole triarylborane
1F
1CN
dyad 1, 1-F and 1-CN.
1
Fig 4. H–NMR titration spectra of bisthiazole triarylborane dyad 1 with
TBAF in CDCl
3
.
The Fig 6 shows the energy level digram of the frontier molecular
orbitals (FMO) of bisthiazole triarylborane dyad 1, 1-F and 1-CN
obtained using computational calculation. The HOMO-LUMO gap
in 1, 1-F and 1-CN is a function of anion groups. In bisthiazole
triarylborane dyad 1 the HOMO-LUMO gap is 2.84 eV. After
In order to understand the anion sensing behaviour of the bisthiazole
substituted triarylborane 1 the time dependent density functional
(
TD-DFT) calculation was performed. The quantum chemical
2
7
calculations were performed using the Gaussian 09 program. The
structures of 1, 1-F, and 1-CN were optimized using B3LYP level
The 6-31G** basis set for C, N, H, B, S used for all the calculations
−
addition of F anion the HOMO and LUMO energy gap was
−
decreased to 0.15 eV. The addition of CN further reduced the
HOMO-LUMO gap to 0.13 eV. The HOMO-LUMO gap in
(
See Fig S1). The solvent calculations were carried out in the
bisthiazole triarylborane dyad 1, 1-F and 1-CN follows the order
tetrahydrofuran (THF) using the polarized continuum model
1
CN > 1F >1.
28
(
CPCM) of Gaussian 09 software.
The 1, 1-F and 1-CN experimental and computed (TD–DFT:
B3LYP) (UV–vis) absorption data are represented in Figure S2. The
strong absorption band of 1, 1-F and 1-CN calculated at B3LYP
levels are 495 nm, 524 nm and 507 nm respectively. The
experimental values for this transition are 416 nm, 428 nm and 503
nm for 1, 1-F and 1-CN respectively. The experimental values are in
good agreement with the TD–DFT/B3LYP values.
In bisthiazole triarylborane dyad 1 the HOMO → LUMO transition
contributes to the lowest excited state by 98% and oscillator strength
is 2.73. After addition of fluoride ion the 1-F shows HOMO-1 →
LUMO+1 transition contribute to the lowest excited state by 30%
and in case of 1-CN shows major contribution (59%) between
HOMO-1→LUMO+1 (Table 1) and these transitions are shown Fig
S3.
Fig 6. The energy level diagram of the frontier molecular orbitals of
bisthiazole triarylborane dyad 1, 1-F and 1-CN calculated using B3LYP level
of TD-DFT theory.
Table 1. Computed vertical transition energies and their Oscillator
strengths (f) and major contributions for the 1, 1-F and 1-CN.
Bisthiazole
TD-DFT/ B3LYP (DCM)
Major contribution (%)
triarylborane
λmax
f
In summary, we have designed and synthesized new bisthiazole
triarylborane based fluorescent chemosensor for the selective
1
495 nm
1.7002 HOMO→LUMO (98%)
−
detection of F and CN anions. The bisthiazole triarylborane
−
−
selective detects F and CN ions the presence of Cl , Br , I , NO
−
−
−
−
−
2
1
1
F
524 nm
507 nm
0.3902 HOMO-1→ LUMO+1 (30%)
1.6259 HOMO-1→LUMO+1 (59%)
ions and exhibits colorimetric and fluorometric response. The
computational and Photophysical properties show strong electronic
communication between the triarylborane and bisthiazole unit. The
CN
−
NMR titration studies further confirmed the selective detection of F
−
and CN anions. The ease of synthesis and simplicity in sensing of
The electrostatic potential surface of bisthiazole substituted
triarylborane dyad 1, 1-F and 1-CN are shown in Fig 5. In 1 the
bisthiazole groups show negative potential (-0.011) whereas the
triarylborane moiety show positive potential (+0.011) confirming
bisthiazole is acting as a acceptor. Since Boron is electron deficient,
−
triarylborane bisthiazole can be used for onsite anion sensing of F
−
and CN anions.
Supporting Information
−
1
the addition of F should increase the electron density on Boron
General experimental methods, synthesis procedures, copies
H
1
3
which was confirmed by the negative potential (-0.019) of
triarylborane unit from the electrostatic potential surfaces of
bisthiazole triarylborane dyad 1F.
NMR, C NMR and HRMS spectra, UV-vis and fluorescence
spectra, TD-DFT graphs and Benesi–Hildebrand plots are provided.
Corresponding Author
*
E-mail: rajneeshmisra@iiti.ac.in
Tel.: +91 7312438754; Fax: +91 7312361482.
Acknowledgements:
The work was supported by DST, and CSIR Govt. of India, New
Delhi. We are grateful to the Sophisticated Instrumentation Centre
(
SIC), IIT Indore.

4
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5
1
2
) Synthesis of Bisthiazole based triarylborane
is reported.
)
The
results are explained by using theoretical
TD-DFT calculations.
−
) The binding constant towards the F and
3
−
CN ion are respectable.

6
TOC